Electroacoustic transducer

ABSTRACT

An acoustoelectric transducer comprising a laser source A and a light receiver H, wherein a soundfield S is provided by which the propagation velocity of the laser beam may be modulated according to the sound pressure while it traverses the soundfield S.

1. SUBJECT OF THE INVENTION

This invention relates to the faithful conversion of acoustic signals (noise, voice and music) into electric signals. The electric signals may then be transmitted or stored by conventional methods. A microphone is introduced, which directly transduces the sound waves into optical and then into electric signals without requiring the aid of movable components such as a diaphragm.

For this purpose the novel microphone uses the influence of sound waves, more precisely, their pressure fluctuations on the light velocity of a laser beam which traverses the medium of the sound field. The change of the light velocity Ac is proportional to the sound pressure p. This small change Ac may be determined by means of an interference assembly and then transduced into an electric signal proportional to the sound pressure. This is the output signal of the novel microphone.

2. PRIOR ART

With the currently used microphones (sound transducers) the sound pressure deflects elastic components such as a diaphragm. The deflection is converted into the electrical measuring signal.

Very popular is the dynamic microphone, where the deflection of the diaphragm induces a voltage within a coil. Nowadays the largest dynamics are achieved with the capacitor microphone, wherein the deflection of the diaphragm causes a change in the capacitance of the capacitor. Since lately there have been microphones available, wherein optical methods (e. g. interference or reflection) are adopted to measure the diaphragm deflection. There are always movable or deflectable parts (diaphragm, moving coil, ribbon, powdered coal) involved.

3. DISADVANTAGES

Mechanical systems have natural vibrations and their deflection is limited whereby the electric output signal is partially falsified. It is difficult to reliably compensate such influences in the large pressure range (audibility threshold: 20 μPa, threshold of pain: 100 Pa) and in the wide frequency range (20 Hz to 20 kHz).

Mechanical systems also respond to structure-borne sound and to air flows, which may cause interfering signals.

Sensitive, precise and low-noise microphones are usually not sufficiently small and thus interfere with the soundfield to be measured.

In electrically measuring systems (capacitor, moving coil) electromagnetic stray fields may affect the output signal.

4. GOAL

What is desired is a sound transducer which converts the sound waves undistorted into electric signals, wherein no movable parts are required. It shall work in the entire audible frequency range and at all loudness levels.

5. SOLUTION

The light velocity in a medium is

$\begin{matrix} {c_{M} = \frac{c}{n}} & (1) \end{matrix}$

-   c: light velocity in vacuum c=3.10⁸ ms -   n: refractive index of the medium.

The refractive index of air at 15° C. and under a pressure of 0.101 MPa is 1.000326 for light having a wavelength of 0.2 μm and 1.000274 for light having a wavelength of 1 μm. Therefore it is larger than the refractive index of 1 in vacuum by 326.10⁻⁶ for UV light and by 274.10⁻⁶ for IR light.

The refractive index also changes with the pressure such as

$\begin{matrix} {\frac{n}{p} = {\frac{0,{3 \cdot 10^{3}}}{10^{5}{Pa}} = {{3 \cdot 10^{- 9}}\frac{1}{Pa}}}} & (2) \end{matrix}$

but depending on the light wavelength. Therefore, also the light velocity changes (Eq. 1) according to:

$\begin{matrix} {{\Delta \; c_{M}} = {\frac{- c}{n^{2}}\frac{n}{p}\Delta \; p}} & (3) \end{matrix}$

For example, the light velocity in air decreases by 0.9 m/s when the air pressure is increased by 1 Pa.

The change in light velocity according to Eq. 3 may be used to determine the sound pressure. Ac of the light beam is proportional to the sound pressure p in the traversed soundfield.

By means of the interference of both halves of a splitted laser beam, this small change in velocity Δc may be determined. In FIG. 1 the design is schematically depicted.

Subsequent to the splitting on the mirror B the one beam is directed through the soundfield S along the path of the length L₁. The other beam travels on the path of the length L₂ through the sound-insulated housing G. Both of the beams interfere behind the mirror C. The detector H determines the intensity of the light and gives a proportional electric signal.

Both of the beams are described by two wave equations:

E ₁ =A cos (ωt−L ₁ k ₁)  (4)

E ₂ =A cos (ωt−L ₂ k ₂)  (5)

-   A: amplitude -   ω: angular frequency ω=2πν; ν: frequency of light -   L₁: path between the mirrors within the soundfield S -   L₂: path within the sound-insulated housing G (note: the remaining     light paths are assumed to be of equal length. Thus they have no     influence on the calculation) -   k₁: wave number in the soundfield

$\begin{matrix} {k_{1} = {\frac{2\; \pi}{\lambda_{1}} = {\frac{\omega}{c_{M} + {\Delta \; c}} = {\frac{\omega}{c_{M}}\left( {1 - \frac{\Delta \; c}{c_{M}}} \right)}}}} & (6) \end{matrix}$

(note: it is allowed to discontinue the progression after the first term, since

$\frac{\Delta \; c}{c_{M}}$

is very small compared to 1)

-   k₂: wave number in the insulated housing

$\begin{matrix} {k_{2} = {\frac{2\pi}{\lambda_{2}} = \frac{\omega}{c_{M}}}} & (7) \end{matrix}$

-   λ₁ and λ₂: wavelengths.

A light intensity I, which is proportional to (E₁+E₂)², is present at the receiver.

Due to the time averaging over one light period the time dependence drops out and for the intensity at the receiver it follows

$\begin{matrix} {I = {I_{0}\left\{ {1 - {\cos \left( {{L_{1}k_{1}} - {L_{2}k_{2}}} \right)}} \right\}}} & (8) \\ {I = {I_{0} - {I_{0}\cos \left\{ {{\frac{\omega}{c_{M}}\left( {L_{1} - L_{2}} \right)} - {\frac{\omega}{c_{M}}\frac{\Delta \; c}{c_{M}}L_{1}}} \right\}}}} & (9) \end{matrix}$

Trigonometric Conversion

$\begin{matrix} {I = {I_{0} - {I_{0}\left\{ {\cos \frac{\omega}{c_{M}}\left( {L_{1} - L_{2}} \right)\cos \frac{\omega}{c_{M}}L_{1}\frac{\Delta \; c}{c_{M}}} \right\}} - {I_{0}\left\{ {\sin \frac{\omega}{c_{M}}\left( {L_{1} - L_{2}} \right)\sin \frac{\omega}{c_{M}}L_{1}\frac{\Delta \; c}{c_{M}}} \right\}}}} & (10) \end{matrix}$

Via the phase difference (L₁−L₂) it is possible to set

$\frac{\omega}{c_{M}}$

to each value between 0 and 2π, wherein multiples of 2π may be added thereto. If the value

$\left( {L_{1} - L_{2}} \right) = {\frac{c_{M}}{\omega}\left( {\frac{\pi}{2} + {z\; 2\; \pi}} \right)}$

is selected therefore (z being an integer), the cosine function disappears.

What remains is only

$\begin{matrix} {I = {I_{0} - {I_{0}\sin \left\{ {2\pi \; \frac{L_{1}}{\lambda}\frac{\Delta \; c}{c_{M}}} \right\}}}} & (11) \end{matrix}$

Here

$\frac{2\; \pi}{\lambda}$

with the wavelength λ takes the place of

$\frac{\omega}{c_{M}}.$

Since the argument of the sine function is very small compared to 1 it may be approximately substituted by its argument.

The decrease in the intensity I₀−I (measured at the receiver) is

$\begin{matrix} {{I_{0} - I} = {I_{0}\frac{2\; \pi \; L_{1}}{\lambda}\frac{\Delta \; c}{c_{M}}}} & (12) \end{matrix}$

It is proportional to the change in the light velocity Δc and to the length L₁ of the light path in the soundfield. Due to Eq. (3) it is then also proportional to the sound pressure {tilde over (p)}. It is this proportionality between sound pressure and change in intensity at the receiver the function of the suggested microphone without a diaphragm is based upon.

6. REFERRING TO THE DRAWING THE INVENTION WILL BE EXPLAINED IN MORE DETAIL BY MEANS OF ONE EXEMPLARY EMBODIMENT

A prototype of a diaphragmless microphone by means of light interference is presently not yet available. However, the principle as it is described under Solution 5 could be verified using an experimental set up according to FIG. 1. A laser diode made of an high performance green laser pointer serves as a radiation source. It is a diode pumped neodymium-yttrium-aluminum-garnet laser (Nd: YAG laser) having a frequency doubling. The wavelength is 532, the output power is max. 5 mW.

The laser was removed from the housing and attached to the optical bench by means of a fixture member. For beam splitting so called beamsplitter cubes were utilized, since they provide a clearer split of the beam in comparison to a semitransparent mirror, i.e. they do not cause any secondary reflection. Moreover, silver plated mirrors are used to achieve a highest possible reflectance. The detector is a photodiode which, having an already integrated preamplifier, provides an output signal of 0.4 A/W (Newport Battery Biased Silicon Pin Detector). The output signal of the detector is supplied to a digital storage oscilloscope (Tektronix TDS220).

An Elac™ speaker being connected to a small amplifier is used as a sound source. The signals are generated through a function generator (KR-Lab Sweep Generator F 47).

For example, three sine signals generated by the tone generator having 500 Hz, 1 kHz und 2kHz, were measured by the diaphragmless microphone and displayed on the oscilloscope as a function of time.

7. ADVANTAGES OF THE INVENTION

-   -   Surprisingly, it is possible even with the experimental form of         the novel microphone to convert sound signals without the aid of         moved parts (diaphragms), thus without mechanics, into electric         signals.     -   Subsequent to the required development the microphone could be         manufactured small, robust and compact. Its influence on the         soundfield would then be small.     -   Since the microphone is operating optically, electromagnetic         interference fields have hardly an influence.     -   The principle of the invention may also be utilized for sound         measurement with other media than air.     -   Thanks to the interference method between the two laser beams,         changes in air pressure (weather, operational altitude) have no         influence. 

1. An acoustoelectric transducer comprising a laser source and a light receiver, characterized in that a soundfield is provided by which the propagation velocity of the laser beam may be modulated according to the sound pressure while it traverses the soundfield.
 2. An acoustoelectric transducer according to claim 1, wherein the variation of the propagation velocity of said laser beam is detectable by interference with a coherent laser beam, wherein the second laser beam travels an approximately equally long distance, however, being protected against the incident sound by a housing, wherein preferably an opening of the housing ensures a pressure equalization with the atmosphere.
 3. An acoustoelectric transducer according to claim 1, the phase difference between the two laser beams is adjustable to λ/4+λ/z, wherein z is an integer.
 4. An acoustoelectric transducer according to claim 1, the two laser beams origin from a pulsed laser beam by a beam splitter, wherein the pulse frequency is above the audible range.
 5. An acoustoelectric transducer according to claim 1, the two laser beams each are reflected multiple times to and fro between two plane parallel mirrors, wherein the one of the mirror pairs and its interspace is exposed to sound whereas the other one is protected against sound.
 6. An acoustoelectric transducer according to claim 1, the pulsed light of the interfering beams falls onto a detector, such as a photoelectric receiver, and is transduced into an electric signal, wherein the ratio between an output signal and noise and interference signals is improved preferably by a lock-in-technique. 